1. Field of the Invention
In particular the invention relates to an instrument that utilizes selected ion flow tube (SIFT), or selected ion flow drift tube (SIFDT) technique which is a fast flow tube/ion swarm method for the study of positive or negative ions with atoms and molecules. A selected ion flow tube can either be a drift tube which has a potential gradient applied to it or a flow tube which has no gradient applied to it. In the following description, the general term flow tube is therefore intended to encompass both forms of technique, that is SIFT-MS and SIFDT-MS.
2. Description of the Prior Art
In SIFT and SIFDT apparatus, the ions are created in an ion source which is external to the flow tube. The ions are then extracted from the ion source by a quadrupole mass filter which acts on the incident ion beam to create a pure species of ion beam (precursor). An electrostatic lens is then used to focus the ion beam which is injected into one end of a flow tube or drift tube which has a flowing carrier gas, usually helium or a mixture of helium and argon or nitrogen. The carrier gas is prevented from entering the quadrupole mass filter by being injected into the flow tube through a venturi orifice in a direction away from the orifice. This enables the swarm of single ion species to be thermalised in a flow tube at the same temperature as the carrier gas flows along the flow tube and quickly establishes a laminar flow of gases through the flow tube. The flow tube or drift tube communicates via a downstream orifice with a downstream chamber housing a quadrupole mass spectrometer system where the ions are mass analyzed and counted.
This form of instrument requires a chamber for the upstream quadrupole mass filter which is connected by the flow tube to a separate generally substantially identical chamber in which the downstream quadrupole mass spectrometer is housed. To allow the quadrupole mass filters to operate effectively, the interiors of both the upstream and the downstream chambers are pressurized at a pressure generally of about 10−6 Torr which is created by individual pumps. The pressure in the flow tube is generally much greater than the pressure in the chambers and generally is in the order of 0.5 to 1.0 Torr.
Because of the requirement of having separate chambers and because of the comparatively substantial size and capacity of the two pumps, a SIFT-MS or a SIFDT-MS instrument is of a substantial size. In addition because of the type of the pumps needed, considerable noise can be created when the instrument is operating. If the instrument is to be made at all portable, it is highly desirable that the instrument including the pumps be housed within a suitably small structure and because of the size and capacity of the pumps it is necessary that considerable attention also be given to adequate sound deadening.